c-s mixed operation

[muzarski]

Motivation

Cassandra-stress supports mixed workloads. It allows to run multiple kinds of operations. Allowed operations are:

• read
• write
• counter_read
• counter_write

The parameters supported by mixed command are:

• all of the common parameters (e.g. used by read)
• all of the counter_write parameters
• ratio() parameter based on which tool samples an operation to execute. E.g. ratio(read=2, write=1) means that there will be approximately 2 read operations per 1 write operation.
• clustering distribution parameter which tells the tool how many times to run the operation that just got sampled.

Changes

• Prepared settings module for MixedOperation
• Implemented MixedOperation

[roydahan]

@muzarski let's try to move this one forward, once we will have this option supported we wlll be able to replace some of the currentl longevities that are using cassandra-stress with cql-stress.

[muzarski]

v2:

• addressed review comments
• rebased on top of https://github.com/scylladb/cql-stress/pull/68. Can be useful to review mentioned PR to see how the changes interact with mixed command.